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package elevenHomework;

/**
 *
 * @author Administrator
 */
import java.io.BufferedInputStream;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
  
public class ProblemC {
  
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
       Scanner cin = new Scanner(System.in);
       Circle c = new Circle(cin.nextDouble());
       System.out.println(c.getArea());
}
}
 
class Circle{
    private double r;
    public static final double Pi = Math.PI;
    public Circle(double r){
        this.r = r;
    }
     
    public double getArea(){
        return 2*Pi*r;
    }
}
